1. Field of the Invention
The present invention relates to a unique titanium-based catalyst system for producing linear low density co-polymers of ethylene. More specifically, the present invention relates to a catalyst comprising titanium and magnesium components. The catalyst is useful for producing ethylene polymers having narrow molecular weight distribution and improved branching compositional distribution.
2. Description of Related Art
Polyethylene polymers are well known and are useful in many applications. Linear low density polyethylene co-polymer (LLDPE), having density of 0.910 to 0.945 g/cm3, is produced primarily using gas phase processes and has been important in commodity and industrial areas over last 20 years. Intense research has been directed to developing high performance LLDPE resins having better impact strength, higher transparency, and less wax content. For example, super strength LLDPE, an ethylene co-polymer having hexene as the alpha-olefin, displays narrow molecular weight distributions and narrow branch compositional distributions. Much effort has been focused on developing new catalyst systems for producing super strength LLDPE having these properties.
Single site catalyst systems employing organometallic compounds such as metallocene compounds are known for controlling molecular weight distributions and branching compositional distributions. Unfortunately, it is difficult to apply these catalysts in existing plants, so the use of such systems in gas phase processes for producing LLDPE has been slow to develop. Also, the solubility properties of organometallic compounds (MAOs) lead to less favorable immobilization processes on inorganic supports, further hindering their applicability.
In contrast, supported titanium-based catalysts (Ziegler-Natta catalysts) can be used in existing processes, without modification. Consequently, there has been intensive development of titanium based catalyst systems for improving molecular weight distribution and compositional distribution of LLDPE.
U.S. Pat. No. 4,748,221 and European Patent No. 0 703 246 A1 describe a common Ziegler-Natta catalyst system. Magnesium metal powder is reacted with butylchloride in a non-polar solvent in the presence of Ti(OR)4. Further treatment with TiCl4/Ti(OR)4/butylchloride results in the formation of a catalyst suitable for gas phase ethylene co-polymerization. These patents use a one step reaction with BuCl, and TiCl4/Ti(OR)4. Also, the LLDPE obtained using this catalyst does not have the narrow molecular weight distribution and compositional distribution that is desired in the newer high performance resins, such as super strength LLDPE.
Other examples of titanium-based catalysts for traditional LLDPE are described in U.S. Pat. Nos. 5,047,468, 5,091,353, and 5,192,731. U.S. Pat. No. 5,047,468 describes a catalyst system for LLDPE production, which is obtained by dissolving MgCl2 with [TiCl3 (AlCl3)1/3] in THF to make a solution containing MgCl2 and titanium halide that is subsequently immobilized on silica support. In U.S. Pat. Nos. 5,091,353 and 5,192,731, MgCl2 is dissolved in electron donating solvent and reacted with alkylaluminum compounds to solidify magnesium halide with aluminum alkoxy compounds. Contacting the solid with titanium halide affords a solid silica catalyst with good morphology and co-polymerization ability. However, the polymer formed with the catalyst results in wide compositional distribution some polymers also with a wide molecular weight distribution.
U.S. Pat. Nos. 5,260,245, 5,336,652, and 5,561,091 describe a catalyst system in which dialkylmagnesium and silane compounds are reacted with —OH group of a silica support which is then contacted with transition metal halide to form relatively homogeneous active site. This catalyst system exhibits more homogeneous ethylene polymerization or co-polymerization capability than traditional magnesium-titanium (IV) halide based catalyst systems. However, this catalyst system requires extra processing steps because the silica support must be treated, either chemically or by heating, to remove bound water and excess —OH groups prior to the formation of the catalyst.
U.S. Pat. Nos. 6,500,906, 6,590,046, 6,841,498, and 6,956,003 describe a catalyst system that does not require immobilization of the titanium component onto the magnesium halide support prior to use. The catalyst system employs a liquid phase transition metal solution in-situ with solid magnesium halide support. The transition metal solution is prepared by reacting Mg[AlR′(OR)3]2 with a nitrogen bound chelating ligand (N-chelate ligand) such as dialkylcarbodiimide, and then with TiX4 as described in where X is halide. This method has the advantage of not relying on a complicated immobilization process to ensure improved catalyst properties.
The preparation methods described in the art for the catalytic control of molecular weight distribution and/or branching compositional distribution are geared toward the complicated task of controlling the active site formation process, which in turn requires careful control of the catalyst precipitation process to ensure consistent catalyst properties. Catalyst properties are deteriorated in the absence of control over the precipitation process. Therefore, a need exists for a simple and efficient process for obtaining a magnesium-titanium based catalyst system for polyethylene polymer and co-polymer suitable for super strength LLDPE with controlled narrow molecular weight distribution and improved branching compositional distribution.